Method for manufacturing a rigid power module suited for high-voltage applications

ABSTRACT

Method for manufacturing a rigid power module with a layer that is electrically insulating and conducts well thermally and has been deposited as a coating, the structure having sprayed-on particles that are fused to each other, of at least one material that is electrically insulating and conducts well thermally, having the following steps:
         manufacturing a one-piece lead frame;   populating the lead frame with semiconductor devices, possible passive components, and bonding corresponding connections,   inserting the thus populated lead frame into a compression mould so that accessibility of part areas of the lead frame is ensured,   pressing a thermosetting compression moulding compound into the mould while enclosing the populated lead frame,   coating the underside of the thus populated lead frame by thermal spraying in at least the electrically conducting areas and overlapping also the predominant areas of the spaces, filled with mold compound.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 12/730,674 filed Mar. 24, 2010, which is entitledto the benefit of German Patent Application No. 10 2009 014 794.2 filedon Mar. 28, 2009, the contents of both are incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a rigid power modulesuited for high-voltage applications.

BACKGROUND OF THE INVENTION

Power semiconductor modules are until now typically formed by multipletransitions from solder, adhesive, LTB (low temperature bonding), or apermanently flexible thermal-conduction paste that connects theconstituent elements like cooling body, heat sink (base plate),substrate, and semiconductor with each other. In the process thesemiconductors have to be soldered onto the substrate that mostlyconsists of several (insulating and metallic) layers, the substrate inturn has to be soldered onto a metal heat sink that, conducting wellthermally (and mostly also electrically), represents the heat spreaderand finally this heat spreader has to be connected to a cooling body. Itis important that the high currents or electrical voltages that are usedto operate the power semiconductors cannot reach the cooling body. It istherefore important to create a safe electric insulation and still toensure a good heat transfer. The substrate is therefore conventionallyformed as a thick-layer substrate, DCB or the like, where a ceramiccore, e. g. Al₂O₃ or AlN or the like as an insulator with good thermalconduction is provided with two structured electrically conductinglayers that consist, for example, of copper or thick film pastes. Aproblem with this type of structure is that the considerable amounts ofheat are conducted through one or more transitions of solder or adhesivethat embrittle by aging; the heat flow then has to be conducted throughcross-sections that are correspondingly smaller and then age evenfaster.

Rigid power modules are protected against the influence of time andageing by encapsulation. A method for manufacturing a rigid power moduleis for example already known from DE 10 2007 020 618 B3 by theapplicant, where a compression mould is used to form a mould compoundaround a lead frame (fixed in the compression mould by a mounting die inthe compression mould) in the populated state, during the formingprocess recesses remain at pre-determined positions and webs of the leadframe can be removed later by the punch that is inserted in therecesses.

The applicant's DE 10 2004 055 534 A1 already discloses a powersemiconductor module whose construction is known having a sprayed-onlayer of particles that are fused together as an electrically conductingwiring plane, intimately connected with this layer, on the sprayed-onlayer. With this module a rigid ceramic substrate is replaced by ametallization. Therefore a solution is sought only for the inside of amodule.

SUMMARY OF THE INVENTION

The present invention therefore is to enable a rigid power module, suchas for automotive applications, in particular in the case of hybridvehicles, to be provided with the necessary firmly bonded link to themodule body and to be precise in a thermally conducting, electricallyinsulating, but in particular also water-proof and water-tight manner.

In the case of the modules described above of the prior art a problemwas the dielectric strength that can be achieved, i.e. its voltageclass.

The lead frame elements which are exposed to the cooling body are tiedto a potential in the case of the methods mentioned above and thereforemust be insulated electrically from each other and relative to thecooling body.

In the case of the prior art, an insulating paste or an insulating filmis placed between the module body with the lead frame elements and thecooling body for this purpose. Lead frame elements are typically spacedapart by 0.5 mm.

Typical heat-conducting films (such as Keratherm (R) Red) possess adielectric strength of around 4000 volts/mm. However, this dielectricstrength is not applicable when no material connection exists betweenmodule and/or lead frame element. In this case the dielectric strengthof the creepage distance between the lead frame elements has to beapplied and it amounts to only 250 volts (according to DIN EN 60664-1)for 0.5 mm in an inhomogeneous electric field. However a paste has nomaterial connection with the mould body and the lead frame. Thereforethe value of the creepage distance must be assumed.

For both cases, pastes and films, the disadvantage therefore consists inthe fact that in the best case a positive matching can be achieved. Thisdoes not extend the dielectric strength as desired.

Furthermore electric insulation of the lead frame elements relative tothe cooling body can be achieved by means of suitable materials (forexample Kapton(R) film), however the heat conduction values thenlikewise required do not exist.

On top of this, pastes possess a technical disadvantage that undermounting-pressure life-long flowing takes place along the smoothsurfaces of module and cooling body. In the case of temperature cycling,the module deformation even leads to the pumping of the paste out of thegap between module and cooling body (the so-called pump-out effect). Thedisadvantage of both options used according to the prior art istherefore a permanent deterioration of the heat resistance toward thecooling body over time.

Finally it has turned out to be a disadvantage that the thermalproperties of pastes and films, even in the case of special materials,are limited to around 0.5 to 5 W/mK. Thus only small heat flows arepossible and the semiconductor is damaged by a high operatingtemperature.

The invention is therefore based on the objective of creating a powermodule that does not exhibit these disadvantages.

According to the invention this is achieved by a method having thefeatures of the main claim. In the process it is also being madepossible that corresponding semiconductors are attached to a lead frameon both sides thermally and electrically by means of soldering, gluing,sintering, wire bonding etc. that are then extrusion coated in atransfer mould process so that the lead frame elements are exposed onthe side to the cooling body.

For electrical insulation this lead frame side is coated in anadditional step by thermal spraying with a layer that insulateselectrically and conducts well thermally and is thus made suitable forhigh-voltage applications.

Using the features of the main claim a thermal connection is thereforeachieved, by a firmly bonded link to the module body on the one hand andby the electric insulation with good thermal conductance of the coolingbody on the other hand.

Further features and advantages of the invention will become evidentfrom the following description of a preferred exemplary embodiment. Inthe drawing

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the inventive new structure that differs from the prior artby an additional ceramic insulation layer,

FIG. 2 shows a conventional structure according to the prior art, and

FIG. 3 shows a structure as in FIG. 1 with an additional metal layersprayed on under the ceramic insulation layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The underside of the modules shown in the drawing are coated with alayer that insulates electrically and conducts well thermally, e. g. anoxidic, nitridic, or carbidic ceramic layer (aluminum oxide, aluminumnitride or silicon carbide). These coating materials, as already listedin the DE 10 2004 005 534 A1 mentioned, are preferably applied bythermal spraying, the layer thickness of the insulator depending on thedesired dielectric strength and the properties of the chosen ceramic orof the ceramic composite.

By clamping with the rough surfaces, in particular their peakroughnesses relative to each other, creep relative to each other and anyform of pump-out effect is prevented, also the thermal-conduction pasteand a heat-conducting film clamping each other well in the rough spots.

As a result a paste can no longer be squeezed out by thethermo-mechanical movement of the module. The rough surface of thesprayed ceramic layer 5 offers a two-dimensional spreading of residualgaps and capillaries that cannot be compressed to zero distance and thusstay completely and permanently filled by thermal-conduction pastes.

Preferred Exemplary Embodiment 1

In the case of a preferred embodiment, as can be seen in FIG. 1 in thefinished state, a mould module with exposed lead frame tracks 4 iscoated, so as to be firmly bonded, with a layer of aluminum oxide in athickness of around 300 μm by thermal spraying. Such a sprayed layer isinsulation resistant up to around 6000 volts.

These layers possess a thermal conductivity of around 24 W/mK andtherefore are thermally significantly more conducting than a paste orfilm.

At the same time as result of the firmly bonded link of the aluminumoxide on the copper of the lead frame 4 and the plastic of the modulebody 1 a firmly bonded link with the sprayed ceramic takes place.

The insulating ability between lead frame tracks increases in the caseof a distance of 0.5 m to 10 000 volts. This enables use in line-voltageapplications where the required minimum dielectric strength amountsto >2500 volts.

Preferred Exemplary Embodiment 2

In the case of a further preferred embodiment (FIG. 3) a mould modulewith exposed lead frame tracks 4 can, in addition to a layer 5 ofaluminum oxide in a thickness of around 300 μm also be coated so as tobe firmly bonded with a further metal layer by thermal spraying.

The at least one further metal layer, for example of Al, Ag, Cu, isapplied to the layer of aluminum oxide, for example by cold gasspraying. Thus the mould module with a layer sequence ceramic—metal isin a position to enable a firmly bonded connection to a metallic coolingbody 6 by soldering or pressure sintering. In this preferred embodimentthe heat flow from module to cooling body through such a firmly bondedconnection is in many cases better than through a paste layer and istherefore to be preferred if applicable.

The reference mark 2 designates the semiconductor, 3 the wire bond thereupon.

While the present invention has been illustrated and described withrespect to a particular embodiment thereof, it should be appreciated bythose of ordinary skill in the art that various modifications to thisinvention may be made without departing from the spirit and scope of thepresent invention.

1. A rigid power module with a layer that insulates electrically andconducts well thermally and has been deposited as a coating, the layercomprising particles of aluminum oxide thermally sprayed onto theunderside of the lead frame, the particles being fused to each other andto the lead frame in at least the electrically conducting areas of thelead frame and extending also over the majority of the area of thespaces between the electrically conducting areas that are filled withmould compound, wherein the layer of aluminum oxide together with afurther metal layer of Al or Ag exhibits a firmly bonded connection to acooling body.
 2. The power module according to claim 1, wherein thefurther metal layer is deposited on a rough outer side of theinsulating, thermally sprayed layer with cold gas spraying.
 3. A rigidpower module with a layer that insulates electrically and conducts wellthermally and has been deposited as a coating, the layer comprisingparticles of silicon carbide thermally sprayed onto the underside of thelead frame, the particles being fused to each other and to the leadframe in at least the electrically conducting areas of the lead frameand extending also over the majority of the area of the spaces betweenthe electrically conducting areas that are filled with mould compound,wherein the carbidic, electrically insulating layer exhibits a thermalconductivity of 24 W/mK and is provided with a thermal-conduction pastethat permanently remains in the residual gaps and capillaries forcreep-resistant positive connection to a cooling body.